1. They don't have webspace and want to put a PDF somewhere online.
2. They do have webspace, but are worried, possibly unnecessarily, that it'll cost them too much money if they send out a link to a big PDF to a large-ish mailing list.
3. For Mac users, where PDFs don't load in a plugin in the browser by default, they want to be able to link to a PDF that opens in the browser instead of popping up an external viewer.
4. And, yes: They have a PDF that is at best gray-area which they want to distribute without hosting it themselves, like a scan of a book chapter for a reading group.